1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
In recent years, liquid crystal display devices have widely been used as flat panel displays for communication tools. This is because the application of the liquid crystal display devices has rapidly been spread in the field of not only OA equipment but also mobile devices due to their enhanced definition, high display quality and increasing lightness and thinness. Under these circumstances, reduction in thickness and weight is still demanded in particular for liquid crystal display panels. A conventional pair of glass substrates of about 1.1 mm in thickness has now been replaced with a pair of glass substrates of about 0.7 mm in thickness. A pair of substrates of smaller thickness has also been considered.
Reduction of the thickness of the glass substrates of the liquid crystal display panel is accompanied by the following drawbacks.
If a pressure higher than a certain level is applied to the surface of the liquid crystal display panel, unevenness in display occurs (stain remains) in a display region. This phenomenon becomes remarkable as the substrate is thinned down. The thinner the substrate is, the more significantly a spacer near the surface which receives a point load is crushed. As a result, a gap between the substrates is reduced near the spacer to a further extent.
For specific explanation of this drawback, as shown in FIG. 14, a pair of glass substrates are arranged to face each other with a plurality of spacers interposed therebetween to leave a gap of 5 μm and a liquid crystal layer is disposed between the substrates. Then, a point load is applied to the middle of one of the substrates. One of the paired substrates is 0.5 mm in thickness, while the thickness of the other substrate is selected from 0.05, 0.1, 0.2, 0.3 and 0.5 mm. As indicated in FIG. 15, the thinner the glass substrate which receives the point load is, the more the glass substrate is warped. Further, as shown in FIG. 16, a 0.05 mm thick glass substrate is warped by 4 μm by a load of only about one-twenty of a load that warps a 0.5 mm thick glass substrate by 4 μm. This indicates that the thinning of the glass substrate easily causes the warpage of the substrate in response to even just a small load.
As a countermeasure against the warpage of the glass substrate, Japanese Unexamined Patent Publication No. 09-073093, discloses a liquid crystal display device including first and second substrates sandwiching liquid crystal therebetween. The first substrate carries a plurality of columnar spacers for leaving a gap between the first and second substrates and a first transparent electrode for applying a voltage to the liquid crystal. The second substrate carries a second transparent electrode for applying a voltage to the liquid crystal. The liquid crystal display device has a peripheral region which does not contribute to the display and a display region enclosed with the peripheral region. As a feature of the liquid crystal display device, the density of the columnar spacers formed on the peripheral region is higher than the density of those formed on the display region. Japanese Unexamined Patent Publication No. 09-073093 describes that this feature reduces display failure due to cell gap failure of the liquid crystal display device.
The technique of Japanese Unexamined Patent Publication No. 09-073093 is directed to restrain the warpage by increasing the number of the spacers to prevent the crush of the spacers and maintain the gap between the substrates. The problem of warpage may possibly be solved by this technique, but another problem of the occurrence of vacuum bubbles in a low temperature environment may arise (bubbles induced by shock at low temperature). At low temperature, the liquid crystal shows particularly significant volumetric shrinkage as compared with other components. Therefore, the spacers cannot follow the shrinkage of the liquid crystal layer and the vacuum bubbles are generated.
Solutions to the problem of the occurrence of bubbles caused by shock at low temperature have been proposed. For example, Japanese Unexamined Patent Publication No. 2002-341354 discloses a liquid crystal display element including liquid crystal sandwiched between two substrates opposing each other with a sealant arranged on the periphery thereof. First columnar spacers are formed on a light blocking region of at least one of the substrates to be in contact with the counter substrate, thereby determining a gap between the substrates. Second columnar spacers are also formed on the same region to have a clearance of 0.2 μm or less between their heads and the counter substrate. With this configuration, the bubbles are not formed by shock at low temperature and unevenness in display caused by a load applied in the production process or an external load is prevented from occurring.
Japanese Unexamined Patent Publication No. 2003-121857 discloses a color filter for a liquid crystal display device provided with columnar spacers including first columnar spacers and second columnar spacers. The first columnar spacers are configured to have a height and a cross-sectional area that make it possible to follow deformation caused by a load applied in a panel assembly process or shrinkage of liquid crystal in a low temperature environment. The second columnar spacers are configured to have a height and a cross-sectional area that make it possible to keep the gap between the substrates even if the liquid crystal is shrunk by a locally applied excessive load or in a low temperature environment. The columnar spacers are adapted to maintain the suitable gap between the substrates in the panel assembly process and are deformed when an excessive load is locally applied or the liquid crystal is shrunk in a low temperature environment, thereby keeping the gap between the substrates uniform. With this configuration, the panel assembly is carried out while keeping the suitable gap. Further, when the excessive load is locally applied or the liquid crystal is shrunk in the low temperature environment, the gap is reduced and kept uniform. Therefore, the liquid crystal display device is achieved without unevenness in color and vacuum bubbles (bubbles formed by shock at low temperature).
According to Japanese Unexamined Patent Publication No. 2002-341354 and Japanese Unexamined Patent Publication No. 2003-121857, a portion that follows the elastic deformation of the gap between the substrates is provided together with the first spacers for keeping the gap between the substrates to prevent the occurrence of bubbles caused by shock at low temperature. Further, the second spacers, which are shorter than the first supports, are provided in parallel with the first spacers. Since a thin liquid crystal display panel is less resistant against a local load and therefore greatly warped, a larger number of spacers must be provided. However, the increase of the first spacers for keeping the gap between the substrates raises the probability of occurrence of bubbles caused by shock at low temperature.
That is, it has been difficult to solve both of the problems involved in the thin liquid crystal display panel such as the warpage caused by the application of local load and the local occurrence of bubbles caused by shock at low temperature.